1. Infrared Spectroscopy
Chemistry 2411 L

2. Introduction
Spectroscopy is an analytical technique which helps determine structure
It destroys little or no sample
The amount of light absorbed by the sample is measured as wavelength is varied

3. Types of Spectroscopy
Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group
Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers
Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns

4. Electromagnetic Spectrum
Examples: X rays, microwaves, radio waves, visible light, IR, and UV
Frequency and wavelength are inversely proportional
c = ln, where c is the speed of light
Energy per photon = hn, where h is Planck’s constant

5. The Spectrum and Molecular Effects
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6. The IR Region Just below red in the visible region
Wavelengths usually mm
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1)
Wavenumbers are proportional to frequency and energy

7. Molecular Vibrations
Light is absorbed when radiation frequency = frequency of vibration in molecule
Covalent bonds vibrate at only certain allowable frequencies
Associated with types of bonds and movement of atoms
Vibrations include stretching and bending

8. IR Spectrum
Baseline
Absorbance/Peak
No two molecules will give exactly the same IR spectrum (except enantiomers)
Simple stretching: cm-1
Complex vibrations: cm-1, called the “fingerprint region”

9. Interpretation Looking for presence/absence of functional groups
Correlation tables
Wade Ch. 12 and Appendices 2A and 2B
Lab book p. 70.
A polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule will absorb weakly or not at all

10. Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:
C-C cm-1
C=C cm-1
CC cm-1 (weak or absent if internal)
Conjugation lowers the frequency:
isolated C=C cm-1
conjugated C=C cm-1
aromatic C=C approx cm-1

11. Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency
sp3 C-H, just below 3000 cm-1 (to the right)
sp2 C-H, just above 3000 cm-1 (to the left)
sp C-H, at 3300 cm-1

12. An Alkane IR Spectrum

13. An Alkene IR Spectrum

14. An Alkyne IR Spectrum

15. O-H and N-H Stretching
Both of these occur around 3300 cm-1, but they look different
Alcohol O-H, broad with rounded tip
Secondary amine (R2NH), broad with one sharp spike
Primary amine (RNH2), broad with two sharp spikes
No signal for a tertiary amine (R3N)

16. An Alcohol IR Spectrum

17. An Amine IR Spectrum

18. Carbonyl Stretching
The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1
Usually, it’s the strongest IR signal
Carboxylic acids will have O-H also
Aldehydes have two C-H signals around 2700 and 2800 cm-1

19. A Ketone IR Spectrum

20. An Aldehyde IR Spectrum

21. O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding

22. Variations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1
The C=O group of an amide absorbs at an even lower frequency, cm-1
The C=O of an ester absorbs at a higher frequency, ~ cm-1
Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency

23. An Amide IR Spectrum

24. Carbon - Nitrogen Stretching
C - N absorbs around 1200 cm-1
C = N absorbs around 1660 cm-1 and is much stronger than the C = C absorption in the same region
C  N absorbs strongly just above 2200 cm-1. The alkyne C  C signal is much weaker and is just below 2200 cm-1

25. A Nitrile IR Spectrum

26. Summary of IR Absorptions

27. Strengths and Limitations
IR alone cannot determine a structure
Some signals may be ambiguous
The functional group is usually indicated
The absence of a signal is definite proof that the functional group is absent
Correspondence with a known sample’s IR spectrum confirms the identity of the compound